The present invention relates to electronic circuits and, more particularly, to a circuit, system, structure and method for controlling and monitoring the speed of a motor.
Electronic circuits are employed in numerous consumer and industrial applications. Such applications are extremely diverse; from computers, to wireless portable communications devices, to industrial controls, etc. In such myriad applications, the electronic circuits, typically fabricated and packaged as integrated circuit (IC) chips, are utilized to achieve various types of functions, for example, digital control, digital-to-analog (D/A) or analog-to-digital (A/D) conversion, mixed signal analysis, etc.
One exemplary circuit application relates to the drive and control of DC cooling fans which are employed in systems such as portable, notebook computers. Such cooling fans operate to cool the various notebook computer components located therein and are controlled using, for example, a DC brushless motor. The DC brushless motor, in turn, is driven and controlled by a control system, as illustrated in prior art FIG. 1, and designated at reference numeral 10.
The exemplary system 10 includes a fan speed controller circuit 12 and a fan driver circuit 14. The fan speed controller circuit 12 provides a control signal, for example, a pulse width modulation (PWM) signal to the fan driver circuit 14 based on a desired fan speed. The system 10 further may include a motor position detection circuit 16 such as a Hall sensor which monitors the relative rotor/stator position of the DC brushless motor 18 and provides motor position information back to the fan driver circuit 14 for commutation control thereof.
The system 10 of prior art FIG. 1 is powered by a system power supply 20, for example a 5 V DC battery. In such systems where circuits such as the fan driver circuit 14 interface with the power supply 20, protection mechanisms such as an external blocking diode 22 may be employed to prevent a reverse bias system condition from causing a reverse current to flow through the fan driver circuit 14 back to the power supply 20.
When providing fan speed control in the system 10 of prior art FIG. 1, prior art solutions often employed two wires between the fan controller circuit 12 and the fan driver circuit 14, respectively. The multiple wires therebetween were used to pass various portions of required data between the fan controller circuit 12 and the fan driver circuit 14, which are typically separate integrated circuit chips (ICs). Using extras wires has several disadvantages since it requires extra IC pins and internal pads associated with the chips. A second prior art solution used an analog feedback methodology. The analog solution, however, requires analog circuitry which may be not be available or suitable in many cases. Lastly, another prior art speed control solution utilized a single wire serial interface, wherein multiple pieces of data of serially multiplexed along the single wire. Such a solution, however, disadvantageously requires an excessive amount of logic overhead to manage and control such data and impacts a driving efficiency of the motor.
There is a need in the art for improvements in fan speed control circuits and systems which overcome the disadvantages associated with the prior art.
The present invention relates to a system and method of controlling the speed of a motor in a variety of applications, for example, fan speed control in portable electronics systems. In particular, the present invention relates to a system and method of generally concurrently controlling and monitoring a motor speed over a single wire via a convolution of motor speed information in a pulse width modulation (PWM) control signal which serves as a speed control signal.
According to one aspect of the present invention, a motor driver system is disclosed in which motor speed control information and actual motor speed information is conveyed over a single wire between system components. The motor driver system comprises a motor controller circuit which is operable to generate a PWM control signal which dictates a duty cycle (and a target speed) at which the motor is to be driven. The system further comprises a motor driver circuit which drives the motor as dictated by the PWM control signal and the desired commutation scheme.
The motor driver circuit is operable to utilize motor position information to draw current associated with the PWM control signal, thereby convolving the motor speed control information with motor speed information over a single wire between the two circuits, respectively, which carries the PWM control signal. Detection circuitry associated with the motor controller circuit detects the current draw associated with the PWM control signal and uses successive detected current draws to determine the actual motor speed. The determined motor speed may then be used to modify the PWM control signal (e.g., vary the duty cycle) to maintain a desired or target motor speed or motor speed range, as may be desired.
According to another aspect of the present invention, a motor driver system having tachometer feedback is disclosed. The motor driver system comprises a motor controller circuit which is operable to generate a PWM control signal which dictates a duty cycle and thus a desired speed at which the motor is to be driven. The system further comprises a motor driver circuit which is operable to drive the motor at a duty cycle associated with the PWM control signal. The motor driver circuit comprises a motor position detection circuit and a current pulse generation circuit. The motor position detection circuit is operable to generate a signal or other type indication which is associated with a predetermined motor rotor position and the current pulse generation circuit is operable to generate a current event such as a current pulse which is convolved with the PWM control signal in response to the rotor position detection signal or indication. The motor controller circuit further comprises a motor speed determination circuit which is operable to utilize the generated current pulses associated with the predetermined motor rotor position to determine the motor speed.
According to still another aspect of the present invention, the current pulse generation circuit comprises a transistor, for example, an NMOS transistor, having a terminal such as a drain coupled to the PWM control signal. The transistor further comprises a control terminal, for example, a gate, which is coupled to the motor position detection circuit. The transistor then conducts for a predetermined time period when the motor position detection circuit provides an indication that the predetermined rotor position has been detected, thereby generating current draw for a predetermined time period which is convolved on the PWM control signal.
According to yet another aspect of the present invention, the motor speed determination circuit comprises a timer circuit which is operable to generate data which indicates a time period between successive current pulses on the PWM control line. The motor speed determination circuit further comprises a decoder circuit which is operable to determine a motor speed based on the data generated by the timer circuit and, alternatively, with information relating to a type of motor being employed in the system. In addition, the motor speed determination circuit may comprise a current pulse detection circuit having a resistive component and a comparator circuit associated therewith, wherein the comparator trips when a voltage across the resistive component exceeds a predetermined threshold, thereby indicating the existence of a current pulse conducting therethrough. The timer circuit may then ascertain a time period between successive comparator trip points at its output.
According to another aspect of the present invention, the motor position detection circuit comprises a Hall sensor which is operable to detect a magnetic flux which varies as a function of rotor position, and output a signal, for example, a differential signal, associated therewith. The motor position detection circuit further comprises a predetermined position detection circuit which takes the Hall sensor output signal and generates a digital signal having a transition frequency which is a function of a predetermined rotor position. For example, the predetermined position detection circuit may comprise a comparator circuit which is operable to trip at a time related to a zero crossing of a differential signal associated with the Hall sensor. The digital signal is then employed to generate current pulses which are convolved with the PWM control signal, wherein the current pulses are associated with a frequency at which the predetermined motor position is detected.
According to still another aspect of the present invention, a method of determining and controlling a motor speed is disclosed. The method comprises driving a motor using a duty cycle which is a function of a PWM control signal. For example, a motor speed controller circuit may output the PWM control signal to a motor drive circuit which employs the PWM control signal to drive the motor using a duty cycle associated with the PWM control signal. The method further comprises detecting a predetermined motor position, for example, by identifying when a rotor of the motor crosses a pole. A current event such as a current pulse is then generated based on the detected motor position and the current event is convolved with the PWM control signal, thereby allowing motor speed information to be conveyed across the same wire which contains the motor speed control information. The method further comprises detecting the generated current event and successive detected current events are employed to ascertain the actual motor speed. Once determined, the motor speed data may be employed to modify the PWM control signal in order to achieve or maintain a desired motor speed target or range.
According to yet another aspect of the present invention, a current pulse is generated and convolved with the PWM control signal by coupling the PWM control signal to a current sink circuit which is activated by the detection of the predetermined motor position. For example, when a rotor of the motor crosses a motor pole, a signal is generated which causes the current sink circuit, for example, an NMOS transistor, to activate and sink current when the PWM control signal is high. Therefore the wire which transmits the PWM control signal contains motor speed control data as dictated by the PWM control signal and actual motor speed data as evidenced by the current pulses convolved therewith.
According to another aspect of the present invention, the current pulses convolved with the PWM control signal are detected by passing the PWM control signal through a resistive component such as a resistor. When a current draw occurs on the PWM control signal, a voltage across the resistive component is detected, for example, using a comparator circuit which trips when a voltage at its inputs exceeds a threshold value. The detected current, for example, the switching of the comparator circuit is then noted using, for example, a timer circuit. By noting the time period between successive current pulses using the timer circuit, a decoder circuit may be employed to ascertain the actual motor speed. For example, a plurality of timer periods between successive current pulses may be accumulated and averaged to identify an average motor speed and/or data associated with the type of motor (e.g., single pole, double pole) may be utilized in determining the motor speed. With the actual motor speed determined, such information may be employed by a motor speed controller circuit as compensation data in varying the PWM control signal to achieve a desired motor speed value or range, as may be desired.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such embodiments and their equivalents. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.